In many applications of household appliances, a heater is required to bring an operating fluid, typically, to the operating temperature required.
Heaters of the above type are typically, although not exclusively, coupled with circulating pumps of the above household appliances.
Such heaters typically include a heating element, normally consisting of a resistance thermometer, and a device for temperature control both in normal operating conditions and in any anomalous or failure conditions. In theory, in order to monitor the temperature of the heater under all operating conditions, a single thermostat would be sufficient. However, to meet some safety principles, such as the presence of a redundant secondary protection in case of failure of the main protection, the thermostat that acts as the main protection must be complemented with a second safety device, typically a thermal fuse.
The thermal fuse is normally used for abnormal and failure condition monitoring and for this reason, it must not intervene in the normal operation of the heater and at the same time, in abnormal and failure conditions it must start promptly before dangerous situations can occur, at the expense of any parts nearby. Laboratory tests show that, in order meet these specifications, the operating temperature of the thermostat must be around 100° C. while that of the thermal fuse is generally around 200° C. In order to meet these requirements, the thermostat is placed on a surface mainly in contact with water and which therefore, under normal operating conditions, is at a temperature of about 85-90° C., while the thermal fuse is placed on a surface mainly in contact with the heating element and which therefore, under normal operating conditions, is at a temperature of about 140-160° C.
If the thermostat and thermal fuse are placed on the same support, this must necessarily be set at a temperature of 85-90° C. to allow the thermostat to perform its function. This would have as a contraindication the slowness of intervention of the thermal fuse, or rather its uselessness, since experimental tests confirm that usually the material thermal fuses are made of, in worst failure conditions, i.e. without circulating water, melts after 10-12 seconds while the thermal fuse would need around 20-25 seconds to intervene.
In order to overcome this drawback, it is known to provide a single support with two distinct zones subject to different temperatures, one reserved to the thermostat and the other to the thermal fuse. A technical solution of this type is for example described in U.S. Pat. No. 7,560,672.
However, such a solution cannot yet be considered optimal. In fact, the presence of a single support implicitly provides for the presence of a thermal bridge between the two zones intended for the thermostat and the thermal fuse, respectively.